In the field of oil and gas exploration, it is usual to drill the well by first drilling the necessary hole, lining this with a steel liner, and backfilling behind the casing with cement. Prior to cementing the casing, drilling takes place under a fluid column of drilling mud. The hydrostatic pressure of the mud prevents flow of fluids from the downhole oil reservoir and/or aquifers into the well. Once the casing is cemented into place, the drilling mud is replaced by a lower density fluid, so that the hydrostatic pressure is less then the natural reservoir pressure. The casing can then be perforated adjacent to the oil producing zone, to allow the oil to flow into the well and to surface.
A perforating gun is used to perforate the well casing. The perforating gun is lowered down the oil well by a wire line to a predefined depth. It contains within it a series of explosive charges which, when detonated, fire projectiles or jets through the lining wail, thus creating orifices (perforations) through which the oil or gas can flow into the wellbore. Typically, a successful perforation will be of the order of 500 mm deep and 5 mm in diameter. Several perforations may be made, at a typical density of between 4 and 10 perforations per 300 mm.
From time to time, an attempt to create perforations is unsuccessful, in that it results in no flow or an inadequate flow. At present, there is no feasible way of investigating the quality of the perforations, so in such circumstances it will not be clear whether the poor flow is attributable to unsuccessful perforation (such as failure of the shaped charges) or to some external factor such as a lower than expected permeability of the rock around the hole.